Method and assembly for coating a moving web

Abstract: A method and an assembly for coating a board or paper web (5) with the help of an air knife (4) as the doctoring means comprising a first backing roll (1) which backs the web during the application of the coating mix to the web (5) and a second backing roll (3) around which the web (5) passes after the coat application and which backs the web during the smoothing of the coat with the help of the air knife (4). The invention is based on applying the coating mix to the web (5) with a correct thickness of the coat by means of a slot-orifice applicator apparatus (2). In the applicator apparatus (2) the coating mix flow is directed in a reverse direction to the travel of the web (5) and only a desired portion of the flow is allowed to adhere to the web (5), whereby the critical doctoring process in the region of the air knife (4) can be easily optimized under varying process conditions.

Description: The present invention relates to a method according to the preamble of claim 1 for coating a board or paper web using an air knife as the doctoring means. The invention also concerns an assembly according to the preamble of claim 3 for implementing said method. When using an air doctor, the coating mix applied to the web is smoothed by directing a high-velocity air jet via a slot-orifice nozzle of the air doctor toward the web. This air knife removes the excess coat from the web surface in the form of a coat mist and this mist is collected into a purpose-designed blow-off hood and recycled back to the coating mix pan. With the help of the air doctor, a smooth coat is attained and the profile of the coated paper or paperboard web follows the contour of the base web. The opacifying power of the applied coat is good. However, this method is not suitable for applying high-solids coats. The greatest drawback of air doctoring is its inherently weak blow-off capability of the excess coat which is further impaired at higher web speed. Consequently, air doctoring must employ coating furnishes of low viscosity and solids content, and yet the usable web speed remains smaller than 500 m/min even in the fastest machines. For these reasons, air doctoring is used almost exclusively in board coating where good opacifying power is imperative and high web speeds are not as critical as in papermaking in general. If the viscosity or solids content of the coating mix is increased, the air doctor looses its ability to blow off the excess coat, and therefore, the finished coat weight becomes excessively heavy. Accordingly, the requirements set for air doctor coating are that the applied coat weight should be as smooth as possible and the weight of the applied coat should give the desired finished coat weight with a sufficiently close tolerance. US-A-3,235,401 discloses an air doctor apparatus in which the web to be coated is taken via a guide roll first to a metering roll of the applicator apparatus. The metering roll is placed in the coating mix pan so that the lower part of the roll is immersed in the pan, while the web runs over the upper part of the roll. The metering roll lifts an excess amount of the mix from the pan to the web which next passes over a rotating predoctoring rod that removes a portion of the excess coat from the web. The purpose of the predoctoring rod is to smooth the coat and remove so much of the excess coat that the air knife can then doctor the coat to the desired finished coat weight. After the predoctoring rod, the web travels onto a backing roll having an air knife adapted close to it so as to blow a narrow-slitted air jet in the reverse direction to the web travel and thus to doctor the coat to its finished weight. Several variants of the above-described type of apparatuses are known in the art, and they constitute the basic construction of air doctors. A drawback of these doctor apparatuses is the rapid decrease of their doctoring performance in terms of coat quality and smoothness at higher web speeds. Patent publication WO-A-9117309 discloses an apparatus further developed from that described above in that the coat quality and maximum usable web speed in coating have been improved. The apparatus described in cited publication is otherwise similar to the apparatus described next above, however, with the exception that the applicator roll is complemented with a doctoring bar which performs both smoothing and metering of the coat transferred from the coating mix pan to the web. In this fashion, the coat applied to the web attains better smoothness and the coat weight is reduced closer to the desired finished coat weight. Such an arrangement has the advantage that the air knife need not remove a great amount of excess coat and the coat will have better smoothness as the applied coat already is relatively smooth to start with. Bar smoothing of the coat being applied to the web improves also otherwise the quality of the end product and permits a higher web speed owing to the reduced blow-off duty of the air knife. Additionally, the use of the rotating predoctoring/metering roll can be obviated. Though the above-described apparatus is capable of overcoming the drawbacks of the air doctor techniques, it still contains several disadvantages mostly related to the applicator roll method. When running at a high web speed, the applicator roll causes strong splashing of the coating mix which then finds its way all around the machinery, on the web and to the surroundings. As the rotational speed of the applicator roll must be strongly increased at higher web speeds, heavy splashing becomes a particular problem at the highest web speeds. When using an applicator roll, uncoated spots will easily remain on the web. Further, the web tension profile has a significant effect on the thickness of the applied coat, and as the air doctor is incapable of smoothing away large variations in coat weights, changes in web tension profile are directly evidenced as quality defects. Moreover, the roll applicator is characterized by an inherent quality defect type, namely, the orange peel pattern caused by the splitting of the coat film at the outgoing side of the contact point between the web and the applicator roll and said orange peel pattern cannot be effectively removed by means of air doctoring, particularly if the web speed is high. A roll applicator cannot be used for applying low coat weights on the web, since mottling of the web by uncoated spots will easily result. Further, the control of the cross-machine profile of the applied coat is rather impossible. FR-A-2 553 305 presents an apparatus which is based on applying the coating mix in a layer onto the web which is backed by a backing roll by means of a slot-orifice coater and an air knife adapted in conjunction with the same backing roll to the end of doctoring the applied coat to a desired weight. Normally the applicator apparatus applies coat in an excess of 20-50 % but, the excess amount can be regulated to zero also. Excess amount can also pass from the entrance side of the applicator apparatus for removing accumulated impurities. US-A-5 104 697 discloses another slot-orifice coater using a predoctoring blade and in some distance behind and in conjunction with the same backing roll a conventional doctor blade instead of an air knife. The linear loading of the predoctoring blade can be regulated by a load control device. The load control device is divided in the cross direction of the web into independent control sections, whereby the loading of the blade can be varied in the cross direction of the web thus allowing for a control means of the applied coating mix quantity in order to obtain a desired coat weight profile in the cross direction of the web. Due to the high feed velocity the excess mix will also spread toward the entry direction of the web. This excess mix is collected to a first mix collecting trough. Furthermore, the excess coating mix flowing over the blade is collected to a second mix collecting trough. It is an object of the present invention to achieve a method and an apparatus offering higher web speed in air doctor coating, and particularly, improved finished quality of the coated web. The invention is based on applying the coating mix in a layer of exactly correct thickness onto the web by means of a slot-orifice coater operating with a counterflow in the reverse direction to the web travel and having an exact control of the mass flow of the coat mix applied to the web to the end of achieving a desired coat weight. More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1. Furthermore, the assembly according to the invention is characterized by what is stated in the characterizing part of claim 3. The invention offers significant benefits. Use of slot-orifice application with reverse flow permits the control of the amount of coating mix applied to the web exactly to the desired coat weight, whereby the coat quantity to be removed by the air doctor remains small. As the amount of removed coating mix is small, the web speed can be increased without compromising the quality of the end product. The machine-direction coat profile remains smooth irrespective of web tension variations, and the cross-direction coat profile can be kept smooth with a narrow tolerance, or alternatively, controlled in a desired manner to take into account the profile variations of the board base web. In other respects, too, the apparatus provides good controllability and it is suited for application of low-weight coats without the hazard of coat mottling. The coat has excellent smoothness as the slot-orifice application employed is capable of applying a high-smoothness coat with a weight very close to the desired finished coat weight. The runnability of the apparatus is good owing to the excellent control facilities offered by the method for optimizing the critical operating parameters of the air knife under widely varying process conditions including web speed variations. In the following the invention will be examined in greater detail with reference to the appended drawings, in which : Figure 1 is a diagrammatic side view of an embodiment of the present invention; and Fig.2 is a cross-sectional view of an embodiment of the applicator apparatus. In the context of this text the term slot-orifice applicator apparatus is used as to refer to such an applicator apparatus in which the coating mix is transferred by direct extrusion via a narrow slot orifice to the surface of a web. Smooth spreading of the coating mix is assured by means of extruding the coating mix on the web at a high speed via a narrow slot orifice. The coater apparatus in fig. 1 comprises a first backing roll 1, an applicator apparatus 2 adapted in conjunction therewith, a second backing roll 3 adapted following the applicator apparatus 2 in the travel direction of a web 5 and an air knife 4 adapted in conjunction with the second backing roll 3. The web 5 passes over the first backing roll 1 through the nip between the backing roll and the applicator apparatus 2 to the second backing roll 3, on which the web further passes through the nip between the second backing roll 3 and the air knife 4. The diameter of the second backing roll 3 can be made smaller than that of the first backing roll, since if the web 5 bends over the backing roll 3 at a smaller radius of curvature, the efficiency of the air doctor in blowing off the excess coat from the surface of the web 5 is emphasized. However, such an arrangement is not mandatory and the design criteria of the roll diameters can be based on different aspects as well. The coat removed from the web surface is collected in a blow-off hood 25. The air knife 4 in the illustrated embodiment comprises an air chamber 4 exiting via a narrow slot orifice 7 extending over the entire machine width and suited for blowing air against the web 5. The slot orifice 7 and the air knife ejected therefrom are aligned in the reverse direction to the travel of the web 5. Since the coating mix dries and its solids content and viscosity increase after its application to the base web due to, among other things, moisture absorption of the web 5, the distance of the air knife 4 from the applicator apparatus 2 is made adjustable to permit the adjustment of the air knife assembly 4 with its backing roll 3 sufficiently close to the application zone as required. Shown in Fig. 1 is an applicator apparatus including a slot orifice 12. This applicator apparatus is adapted in conjunction with a rotating backing roll 1 around which the web 5 to be coated passes. To the underside of the backing roll 1 is sited an applicator extending over the entire cross-machine width of the web 5 and having its framework formed by a support beam 9 with an approximately triangular cross section. Via a feed channel, which extends over the entire cross-machine width of the web 5 along the support beam 9 on the incoming side of the web, the coating mix is fed into a chamber-like space 10, wherefrom the coating mix under pressure flows to the web via a narrow, flat slot-orifice channel 11 extending over the entire web width. The smoothing/premetering blade depicted at the opening of the channel 11 is not part of the invention. The coating mix is fed at a high speed in excess of 1 m/s, whereby an excess portion of applied mix will also be overflown in the reverse direction to the travel of the web 5 past an upper lip 17 of the orifice channel 11. This excess mix is particularly important to the successful outcome of the coating process as its role is to assure a smooth and homogeneous coat. The excess mix reverse flow 14 also permits an extremely accurate control of the amount of coating mix applied to the web 5 as well as the adjustment of the coat thickness down to very thin coats. The return flow 14 of the excess coating mix is collected in an overflow trough 15. With reference to Fig. 2, the slot-orifice applicator apparatus shown therein comprises an upper lip 17 and a lower lip 23. The slot orifice 12 of the applicator is formed by the rounded tip of the upper lip 17 and conformingly curved part of the lower lip 23. The path of the coating mix flow starts as a narrow fat channel 11 which tapers toward the slot orifice 12. The width of the channel 11 at its entrance is approx. 0.5 - 10 mm, typically in the range of 1.5 - 4 mm. Naturally, the length of the channel 11 in the cross-machine direction must extend at least over the entire width of the web. The width of the orifice slot 12 is typically in the range of 0.5 - 10 mm, however, so that at its exit the slot is slightly tapered relative to the inner width of the channel 11. The gap distance from the slot-orifice applicator apparatus to the backing roll 1 (the web) is typically in the range of 1 - 20 mm, most advantageously approx. 3 - 8 mm. The gap distance can be adjusted by moving the lower lip by means of an adjustment apparatus 24. Also the upper lip can be made transferrable relative to the coater framework, whereby also the width of the slot-orifice channel 11 can be made adjustable if desired. The rounded tip of the upper lip 17 invokes a so-called Coanda effect, whereby the coating mix jet tends to follow the surface of the upper lip 17 in the exit of the orifice slot and the coating mix jet is thus aimed in the reverse direction to the web travel. The radius of curvature at the tip may vary in the range of 1 - 50 mm, typically the radius of curvature is selected to be in the range of 3 - 10 mm. A basic precondition to the formation of a suitable jet flow of the coating mix is that the surface of the lower lip 23 is curved at the slot orifice 12 toward the reverse direction with respect to the web travel, whereby the desired aiming for the coating mix jet is attained. In the present apparatus the amount of coating mix feed can be adjusted in multiple different ways, the most important of which is the control of the coating mix flow rate by means of adjusting the volume rate of fresh coating mix pumping. Simultaneously or alternatively, the width of the slot orifice 12 or the jet direction can be varied. The jet direction can be altered by, e.g., rotating the applicator apparatus with its support beam in the same manner as the angle of the doctor blades is adjusted. Such a slot-orifice coating apparatus is described in greater detail in FI patent application 924,841. The coater assembly of the above-described type is operated as follows. The incoming web to be coated passes around the backing roll 1 of the applicator apparatus on which the top side of the web is coated with a coat thickness approximately corresponding to the desired coat weight using a slot-orifice applicator 2. The coat thickness is herein adjusted so that the air knife 4 can smooth the coat at the normal web speed, coat solids and coat viscosity employed to the desired finished coat weight. Obviously, when running with higher web speeds and, e.g., coating mix viscosities, the applied coat thickness must be closer to the finished coat weight than when running at lower web speeds. Yet, the applied coat must be thicker than the finished coat to leave the air knife 4 at least some excess coat to blow off in order to control the coat to its finished weight. If the initially applied coat remains excessively thin, its quality will suffer as the doctoring effect of the air knife 4 remains unusable at least partially and the finished coat weight will not meet the specifications. In the above description example has been given on applicator apparatus suited for use in conjunction with the present invention. Hence, the construction of the applicators can be varied provided that successful application outcome is ensured by using such an applicator apparatus in which the coating mix jet flow is arranged to run in the reverse direction to the web travel and from the slot orifice is applied to the web in the travel direction of the web only such a coat thickness which is essentially equal to the coat thickness corresponding to the desired finished coat weight. Obviously, the present assembly and method are also suited for coating other similar materials besides board and paper.

Claims: A method for coating a moving web (5) such as a board or paper web, comprising taking the web (5) to a backing roll (1) and applying to the web surface a coat by means of a slot-orifice applicator (2) so that from the slot-orifice (12) directly to the gap between the slot orifice and the web is ejected a coating mix flow which is permitted to adhere to the web (5) so as to form the applied coat, in a layer with a weight very close to the finished coat weight, whereas excess mix will spread in a reverse direction to the travel of the web (5), and doctoring said applied coating mix by means of an air knife (4) adapted in conjunction with a backing roll (1,3), characterized by operating the slot-orifice coater (2) with an excess of mass flow of the coating mix and directing said coating mix flow as a jet in the reverse direction to the web travel and altering the jet direction to the end of achieving the desired coat weight, very close to the finished coat weight, after which the web (5) is taken to a second backing roll (3) for doctoring the coating mix by means of an air knife backed by said second backing roll (3). A method as defined in claim 1, characterized by varying the local flow rate of the coating mix flow against the web (5) in a controlled manner over the cross-machine direction of the web. An assembly for coating a moving web (5) such as a board or paper web, said assembly comprising at least one backing roll (1) around which the web (5) is adapted to pass, a slot-orifice applicator (2) for applying the coating mix in an amount very close to the desired coat weight directly to the web in the gap formed between said slot-orifice (12) and the web (5), and an air knife (4) adapted in conjunction with a backing roll (1,3) beside the web (5) to the end of doctoring the applied coat, characterized in that said applicator apparatus (2) comprises means (8) for directing the coating mix flow in the reverse direction to the travel of the web (8) including a slot orifice (12) directing a jet in the reverse direction to the web travel and means alter to the jet direction to the end of adjustment of the coat thickness very close to the desired coat weight, whereby the coat quantity to be removed by the air doctor remains small, and a first (1) and a second backing roll (3) related to said slot orifice applicator (2) and to said air knife (4) respectively. An assembly as defined in claim 3, characterized in that said applicator apparatus comprises an upper lip (17) adapted to the immediate vicinity of the web (5), and a lower lip (23), which is placed close to the upper lip (17) essentially noncontactingly outdistanced from the web (5), whereby said upper lip (17) and said lower lip (23) form a slot orifice (12) suited for applying a flow of coating mix to the web (5) and said slot orifice (12) has a rounded shape at the tip of said upper lip (17) and the shape of said lower lip (23) is curved to essentially confirmingly follow the rounded shape of said upper lip (17) and is extended at the slot orifice (12) toward the incoming direction of the web (5). An assembly as defined in claim 4, characterized in that said applicator apparatus (2) incorporates a control means (24) for adjusting the position of the lower lip (23) and the width of the slot orifice. An assembly as defined in claim 4 or 5, characterized by means for rotating said applicator apparatus to the end of controlling the direction of the coating mix flow.

Inventors: KOSKINEN JUKKA, PALOVIITA PETRI, KOSKINEN, JUKKA, PALOVIITA, PETRI
Applicants: METSO PAPER INC, METSO PAPER, INC.
Application year: 1994
Main classification: D21H25
IPC codes: B05C5, D21H23, B05D1, B05C11, B05D7, D21H25